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Contingencies (Narrative) (Detail) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 3 Months Ended 9 Months Ended 1 Months Ended
Apr. 30, 2011
May 31, 2009
Jan. 31, 2013
Jan. 31, 2012
Apr. 30, 2011
Apr. 30, 2010
Apr. 30, 2009
Jan. 31, 2009
Jan. 31, 2013
Jan. 31, 2012
Nov. 13, 2012
D
Apr. 30, 2012
May 31, 2011
Jan. 18, 2011
Jan. 30, 2009
Dec. 20, 2000
acre
Sep. 30, 2011
Waste Stream Inc [Member]
Nov. 30, 2011
Motors Liquidation Trust [Member]
Contingencies And Commitments [Line Items]                                    
Disputed invoice amount                         $ 3,195          
Aggregate finance asserting liability total                           3,852        
Taxation payable                           2,220        
Penalties and interest payable                           1,632        
Scrap yard and solid waste transfer station                               25    
Undiscounted costs associated with implementing preferred remedies   10,219                                
Present worth cost associated with implementing preferred remedies 12,130                                  
Enter into a Consent Order with DEC                     60              
Environmental remediation charge         549 335 1,532 2,823                    
Percentage of environmental remediation cost             15.00%               30.00%      
Environmental remediation liability     5,313           5,313     5,210            
Accretion expense     34 34         103 103                
Percentage of risk free interest rate     2.70%           2.70%     2.70%            
Future remediation costs                                 $ 3,000 $ 100